Ocean Thermal Energy Conversion (OTEC) systems have been studied and small scale units have been tried since the late 1920's and early 1930's based on the principles of the Rankine Cycle developed in the mid-19th century. The systems involve a turbine having an upstream side and a downstream side. Warm liquid under a partial vacuum is converted into a vapor on the upstream side of the turbine with the vapor pressure being controlled by the temperature of the warm liquid. A condenser is situated on the downstream side of the turbine to cause the vapor, after passing through the turbine, to undergo a phase change back to a liquid. The condenser is coupled to a source of a cooling liquid, and the pressure of the vapor on the downstream side of the turbine is determined by the temperature of the cooling liquid. Small temperature differentials cause small pressure differentials which limit the effectiveness of the turbine.
To increase the temperature differential, OTEC systems are often located where deep water is available and temperature of the ocean surface is at its highest with the most hours of available sunlight. Typically, the most propitious sites are near the equator. To further increase the temperature differential, typical OTEC systems utilize water pulled up from extreme ocean depths of about 1000 meters as the cooling liquid. However, typical OTEC systems utilize about 80% of the energy that they can generate to pump warm surface water and to pump cooling water up from the extreme depths.
A typical prior-art land-based open loop OTEC system 10 is shown in FIG. 1 adjacent to a body of water. The system 10 has a warm water inlet 14 close to the surface 40 of the body of water, where the water typically has a temperature t2 of approximately 27° C. A cold water inlet 12 is provided at a depth of approximately 1000 meters, where the water typically has a temperature t1 of approximately 5° C. Warm water is brought in through inlet 14 and pumped into a warm water degassing tank 16. Air is pumped from the evaporation tank 18 by means of a vacuum pump 22 to achieve a partial vacuum. Degassed warm water is then pumped from the degassing tank 16 into the evaporation tank 18, which results in a portion of the degassed water flashing into steam, thus increasing the pressure in evaporation tank 18 to an initial pressure P1. The steam is then fed into an upstream side of the turbine 26. The steam then passes through the turbine 26 to the downstream side of the turbine and into condenser 20. The condenser 20 is maintained at the temperature t1 by virtue of being fed with the cold water being pumped by pump 24 up from cold water inlet 12. Condensation of the steam in condenser 20 causes a drop in pressure in the condenser 20 and on the downstream side of turbine 26 to an exhaust pressure P2, which is lower than the initial pressure P1. The pressure difference ΔP=P1−P2 causes the flow of steam through the turbine 26 causing the turbine 26 to spin. Turbine 26 then drives electricity generator 28. Water in the form of the condensed steam can be withdrawn from condenser 20 and stored in a tank 30 for use as potable or distilled water. The electricity generator 28 can be connected to a suitable grid 42 for distribution and use.
Major difficulties have prohibited prior art OTEC systems from achieving commercial success. A first difficulty lies in deploying and maintaining a large pipe 10 meters in diameter and 1000 meters deep against the shedding currents typically found in the ocean. A second difficulty lies in the inherent inefficiencies of typical systems. An analysis of the Carnot efficiency with the temperature change Δt=t2−t1 of 20° C. shows that the very best a system could achieve is around 7% efficiency. In practice, with low pumping pressure differentials ΔP between the turbine inlet and outlet (approximately 0.4 psi), pumping losses, and small Δt's, current OTEC systems run between about 1% and 2% efficiency.
Another problem for many of the systems attempted to date is that they are land-based systems where the deep ocean cold water had to be pumped not only up from the depths but it also had to be pumped a considerable lateral distance to reach the onshore plant which causes pressure losses and warming of the cooling water. Ocean front land can be prohibitively expensive and often are at risk due to hurricanes. Areas of highest temperature differential (equatorial Atlantic Ocean) are not close to areas requiring large quantities of energy. Bio-fouling of the entire system has also caused major failures in the past.
What is needed is alternative apparatus that can effectively and efficiently utilize the natural temperature differentials exhibited by selected areas of the ocean to generate a consistent level of usable power.